The invention relates to an apparatus for adapting the speed of carriages which can be moved along a rail arrangement, in particular for nippers which can be made to travel along a rail arrangement for the continued transporting of a moving material web, preferably in a plastic drawing installation.
For example in longitudinal or simultaneous drawing installations for plastic film webs, the film web to be drawn is gripped in the run-in region by nippers which are provided on the left and right sides of the moving material web and are advanced along on the circulating guide track with increasing acceleration and increasing spacing from one another. In the so-called run-out zone after ending the stretching operation, the nippers release the edge of the plastic film web and return on a circulating track to the run-in region.
In the run-out region in particular (but not only there), there is the problem that the individual nippers have to be braked again to a lower speed.
Such a problem arises, for example, in a simultaneous drawing installation known from U.S. Pat. No. 5,072,493, in which the nippers or nipper carriages are driven and made to travel along a guide rail system by means of linear motors. In particular at the transition from the return to the run-in reversal, in this case the individual nipper carriages, which can be made to travel separately and independently of one another by means of a linear motor drive, are switched over again to a different speed level.
Specifically in the case of nippers driven by linear motors, it is possible in principle to perform the desired braking action, for example in the run-out region (onto which the nippers are thus returned again to the run-in region on a curved track), likewise by means of the linear motors provided. In such a case, the linear motors could be used on the operating principle of a hysteresis motor. In this case, the linear motors located in the return push the nippers (which bear a magnetic material with hysteresis) through the run-in reversal, in the region of which no drive is provided. The arrangement, simple in itself, has the disadvantage that the nippers can no longer be influenced in their movement sequence over a relatively long line, and that magnetic material with hysteresis properties has to be additionally applied to the nippers. The forces which can be generated by the hysteresis effect are low, for which reason there must be a relatively long driving line in order to reach the final forces required.
For this reason, it has been proposed to provide a wheel having a friction lining on the circumference at a run-out reversal in such a way that the nippers are pressed against the friction wheel and, due to the frictional engagement, follow the movement of the wheel in the circumferential direction. However, a disadvantage of such mechanical device is the inevitable wear at the friction point, which increases in particular with the magnitude of the difference in speed between the speed of the nipper and the circumferential speed of the wheel. Further, an additionally high stress occurs at the contact point of nipper and running rail. In the case of a change of friction lining, the entire installation must be switched off.
In a further installation, known from U.S. Pat. No. 3,247,544, the nippers are positively taken along by radial chambers which are located on a wheel. The speed changes due to a radial movement on the wheel rotating at constant speed. Here too, the speed adaptation thus takes place mechanically, the wear occurring in this arrangement at the contact point between the chambers and the nipper. It is not possible to compensate for different spacings between nippers. The differences in the speeds between the wheel and the nipper in this case result in collisions at the run-in.